container: make hostconfig.json non-world-readable (0600)
container: make hostconfig.json non-world-readable (0600)

When writing container's `hostconfig.json`, permissions were set to 0644 (world-
readable). While this is not a security concern (as the `/var/lib/docker/containers`
directory has `0700` or `0701` permissions), there is no real need to have these
permissions, as this file is only accessed by the daemon.

Looking at history for file permissions;

- 06b53e3fc7aca2b3dae32edab08c7662d3e9e7e8 (first implementation) used `0666` (world-writable)
- cf1a6c08fa03aa7020f8f5b414bb9349a9c8371a refactored the code, and removed explicit permissions
- ea3cbd3274664f5b16fce78d7df036f6b5c94e30 introduced atomic writes, and brought back the `0666` permissions
- 3ec8fed7476704f061891d4c421c615da49e30c7 removed world-writable bits, but kept world-readable

This patch updates the permissions to `0600`, matching what's used for `config.v2.json`,
which was updated in ae52cea3ab46e1e728606349fb6baa9a8203f3ed, but forgot to update
`hostconfig.json`.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

moby/moby,moby/moby,djs55/docker,thaJeztah/docker,mahak/docker,mahak/docker,cpuguy83/docker,trebonian/docker,yosifkit/docker,konstruktoid/docker-upstream,AkihiroSuda/docker,AkihiroSuda/docker,flavio/docker,kojiromike/docker,moul/docker,gnailuy/docker,yosifkit/docker,flavio/docker,trebonian/docker,cpuguy83/docker,nawawi/docker,bearstech/moby,konstruktoid/docker-upstream,konstruktoid/docker-upstream,nalind/docker,gnailuy/docker,moby/moby,thaJeztah/docker,vdemeester/docker,yosifkit/docker,jameszhan/docker,jimjag/docker,Ramzec/docker,nawawi/docker,thaJeztah/docker,kojiromike/docker,jameszhan/docker,bearstech/moby,vdemeester/docker,Mokolea/docker,paralin/docker,docker/docker,paralin/docker,docker/docker,djs55/docker,jthelin/docker,Mokolea/docker,docker/docker,flavio/docker,vdemeester/docker,cpuguy83/docker,jimjag/docker,trebonian/docker,Ramzec/docker,jimjag/docker,thaJeztah/docker,AkihiroSuda/docker,mahak/docker,mahak/docker,jimjag/docker,konstruktoid/docker-upstream,gnailuy/docker,kojiromike/docker,cpuguy83/docker,nawawi/docker,gnailuy/docker,nalind/docker,nalind/docker,paralin/docker,djs55/docker,trebonian/docker,Ramzec/docker,moul/docker,moby/moby,Mokolea/docker,paralin/docker,bearstech/moby,nawawi/docker,jameszhan/docker,Mokolea/docker,nalind/docker,Ramzec/docker,moul/docker,flavio/docker,jthelin/docker,bearstech/moby,AkihiroSuda/docker,yosifkit/docker,moul/docker